1. Field of the Invention
The present invention relates to a spread spectrum communications system, and more particularly to a spread spectrum communication system of the direct sequence type. The present invention includes a synchronous control, a modulation/demoudulation system and radio communication in spread spectrum communications.
2. Discussion of the Background
In general, a spread spectrum communications apparatus of the spread spectrum communication system of a direct sequence type correlates a spread signal (Pseudo Noise code) of a transmitting signal with a receiving signal so as to synchronize the transmitting signal with the receiving signal. The resulting auto correlation value is made a control signal for the control of the clock frequency for a PN code. The PN code is a spread signal of a transmitter side of the apparatus. Japanese Patent Laid Open Publication No. 1-228338 discloses an example of a code synchronization circuit. The code synchronization circuit is a so called delay locked loop type circuit. The delay locked loop type circuit correlates each spread signal with a received signal at two points in a code sequence on the receiving side, and the control signal is a differential signal between both the correlation value determined with the received signal and the spread signal. This control signal controls the frequency of the clock generator for code and synchronizes each spread signal with a receiving signal in the receiver side.
In addition, practically all spread spectrum communications systems of the direct sequence type handle positive and negative binary numbers. However, the code sequence is not necessarily defined by a binary number, if the code sequence is given the necessary auto correlation characteristics. For example, in `The Merits and Applications of DS-SS Systems Using RZ-codes: FUKUYAMA, TADOKORO, The Institute of Electronics Information and Communication Engineers, SSTA 89-16 pp. 33.about.37, Aug. 4th.about.5th 1989` use of a return zero (RZ) code that outputs zero only a required number of times within one code of a PN code is disclosed. The PN code itself obtains a positive and a negative value and so the entire PN code sequence consists of three values, positive, negative and zero. However, one PN code can consist of only two values. The above literature discloses the spectrum characteristics and the auto correlation characteristics of RZ codes. Also, in Code Division Multiples Access Manchester Coded M-sequence: HABUCHI, HASEGAWA, HAKURA, The Institute of Electronics Information and Communication Engineers, SSTA 89-27, pp. 101.about.104 Aug., 4th.about.5th 1989, the correlation characteristics due to Manchester code are disclosed.
The binary codes that are expressed in M-sequences are generally used for both transmitting and receiving using the PN code used in spread spectrum communications. However, this PN code is used for obtaining the necessary auto correlation characteristics for synchronization in the receiver. The PN code itself can be of any type as long as the necessary characteristics are obtained. These necessary auto correlation characteristics for synchronization are expressed by a linear equation of auto correlation characteristics that pass through the origin of the synchronization points in the synchronization system, and a point in the vicinity thereof, and the auto correlation characteristics are such that they are close to zero for another point. The auto correlation characteristics in a synchronous system reach a peak value nearby the synchronization points, and reach a zero value near an auto correlation characteristic point thereof.
Return-zeroed PN code for turning off the output of the PN code for a constant time, and a Manchester coded PN code have been proposed as variations of the above described PN code. In particular, the Manchester coded PN code enables the circuit to be easily constituted, and is advantageous in synchronous systems.
On the other hand, with the Manchester coded PN code, the linear region becomes smaller when digital modulation of the transmitting signal is performed and when the PN code is used simply to synchronize the transmitting signal (PN code) with the receiving signal. Accordingly, the large auto correlation output consumes much energy at points other than those in the vicinity of the points of synchronization, and there is also a high level of interference signals with respect to other types of communications.
One example of a multiplexing method used in conventional SS communications is that previously described in The Merits and Applications of DS-SS Systems Using RZ-codes: FUKUYAMA, TADOKORO, The Institute of Electronics Information a Communication Engineers, SSTA 89-16 pp. 33.about.37, Aug. 4th.about.5th 1989. This literature describes a method by which a plural number of PN codes for a plural number of data are prepared, each of the data are multiplied by respectively different PN codes, the codes are multiplied and spread, and the spread signals are transmitted simultaneously. However, this method requires many PN code generators and multiplexers.
On the other hand, these is also a system disclosed in `Parallel Spread-Spectrum Communications with Rejection Techniques of Co-Channel Interference,` SATO, The Institute of Electronics Information and Communication Engineers, SSTA 90-39, Oct., 25th.about.26th, 1990. This disclosed system uses RZ code that zeroes the output by a constant proportion within a single chip of a PN code, and that can be used for spread spectrum (SS) communications in the same manner as a normal PN code.
In this manner, the multiplex method of conventional SS communications can be performed as described above, by code-division multiplexing. Multiplexing in SS communications involves the use of a plural number of different PN codes, each PN code being multiplied by a respectively different data signal and transmitted in parallel. However, with this method, it is necessary to have many code generators and multiplexers so that the circuit configuration becomes complex. In one method for avoiding this problem, data signals are multiplied by signals that each have a different phase of one PN code, and then transmitted in parallel. In another method, return-zeroed PN code is used. This method has a state where there is zero output and so power consumption is less and radiated noise is reduced. However, when the output of the circuit is zero such an output is the same as if the circuit is not operating at all and so this non-operating time represents a waste of time.
Moreover, the previously proposed `Japanese Patent Laid Open Publication No. 240266-1990` discloses the use of mutual auto correlation between a normal PN code and Manchester-coded PN code, and realizes a simple demodulation method using the clock pulse auto correlation.